If next week your total site serp referrals goes up (from 'minor updates' that is, or between 'sub-major updates' not from tonights 'major update'), that will likely continue for the next three weeks. If they go down, they will continue downward until the next 3 weeks until the next 'major update'.
What I mean is that rankings tend to increase for a site over the 3 weeks between major updates or decrease - but not both (well, possibly stay constant). For some volitile sites it will make your traffic chart look like a sawtooth wave with a period of 6 weeks (two 3-week cycles).
It is as though the trend in ranking is pre-determined at week 1 of the three week cycle - that is, they are just MIXING in results to the current results (or averaging rankings between the current and the new) in order to make a gradual change.
NOTE: This only applies for large sites ( > 5,000 uniques a day) with great keyword diversity. Smaller sites I've seen have a much more random pattern.